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| United States Patent |
5,114,987
|
|
Cakmak
, et al.
|
May 19, 1992
|
Foamed polymer blends
Abstract
A process for preparing foamed polymer blends of thermoplastic polymers
with elastomeric polymers comprises mixing the elastomeric polymers with a
chemical blowing agent and compounding additives. The mixture is then
combined with a thermoplastic polymer to form a blend, and thereafter
mixed with a curing system and a blowing agent activator. The curable
composition thus prepared can subsequently be blown in either a one or
two-step process. The two-step process entails partially curing the
elastomer in the composition at a lower temperature, and then blowing the
composition at a higher temperature. The one-step process involves (a)
heating the composition to the higher temperature initially, thereby
initiating curing, and after an activating induction period of the blowing
agent, producing blowing; or (b) curing and blowing the composition by
extruding it through an extruder. In both processes, the elastomer present
in the blend is sufficiently cured prior to blowing to prevent the escape
of the blowing bubbles, but not to the extent that bubble formation is
prevented.
| Inventors:
|
Cakmak; Mukerrem (Monroe Falls, OH);
Dutta; Anit (Wilmington, DE)
|
| Assignee:
|
Edison Polymer Innovation Corp. (Broadview Heights, OH)
|
| Appl. No.:
|
597431 |
| Filed:
|
October 15, 1990 |
| Current U.S. Class: |
521/84.1; 521/134; 521/139; 521/140 |
| Intern'l Class: |
C08J 009/10 |
| Field of Search: |
521/134,140,139,84.1
|
References Cited
U.S. Patent Documents
| 3981830 | Sep., 1976 | Takeuchi et al.
| |
| 4166890 | Sep., 1979 | Fried et al.
| |
| 4247652 | Jan., 1981 | Matsuda et al.
| |
| 4442233 | Apr., 1984 | Lohmar et al.
| |
| 4460748 | Jul., 1984 | Rauer.
| |
| 4510031 | Apr., 1985 | Matsumura et al.
| |
| 4519963 | May., 1985 | Yoshida et al.
| |
| 4607059 | Aug., 1986 | Kmiec et al.
| |
| 4680317 | Jul., 1987 | Kuhnel et al.
| |
| 4701472 | Oct., 1987 | Koebisu et al.
| |
| 4721591 | Jan., 1988 | Cheng-Shiang.
| |
| 4766159 | Aug., 1988 | Kitagawa et al.
| |
Primary Examiner: Foelak; Morton
Attorney, Agent or Firm: Hochberg; D. Peter, Kusner; Mark, Weisz; Louis J.
Claims
What is claimed is:
1. A process for preparing a foamed polymeric material from a blend of a
thermoplastic polymer selected from the group consisting of a polyolefin,
polystyrene, poly(methyl methacrylate) and poly(vinylidene chloride) with
an elastomeric polymer selected from the group consisting of natural and
synthetic rubbers comprising the following steps:
(1) mixing a chemical blowing agent and rubber compounding additives with
said elastomeric polymer to form a mixture;
(2) mixing a mixture with said thermoplastic polymer at a temperature at
which at least one of said polymers is in a fluid state, thereby forming a
homogeneous blend;
(3) mixing said blend with a curing system to form a curable composition;
and
(4) activating said curing system and said blowing agent so that the
blowing of said material commences no sooner than when curing commences
and before said elastomeric polymer has been fully cured.
2. A process according to claim 1 wherein the ratio of said elastomeric
polymer to said thermoplastic polymer, on a weight basis, is from about
1:1 to about 3:1.
3. A process according to claim 1 wherein in the absence of a chemical
blowing agent activator, said blowing agent must be heated to a
temperature higher than that of step (2) in order to be activated.
4. A process according to claim 3 in which a chemical blowing agent
activator is added during step (3).
5. A process according to claim 1 wherein said curing system and said
blowing agent are heated to their activation temperature substantially
simultaneously.
6. A process according to claim 1 wherein said activation takes place as
said material is processed through an extruder.
7. A process according to claim 1 wherein said curing system is activated
and said material is partially cured before said blowing agent is
activated.
8. A process according to claim 1 in which said thermoplastic polymer is a
polyolefin, and said elastomeric polymer is an olefin-based elastomer.
9. A process according to claim 5 in which said thermoplastic polymer is
polypropylene, and said elastomeric polymer is an ethylene-propylene
terpolymer.
10. A process for preparing a foamed polymeric material from a blend of a
polypropylene and an ethylene-propylene terpolymer comprising:
(1) mixing a chemical blowing agent and rubber compounding additives with
said ethylene-propylene terpolymer to form a mixture;
(2) mixing said mixture with polypropylene at a temperature at which at
least one of said polymers is in a fluid state, thereby to form a
homogeneous blend;
(3) mixing said blend with a curing system to form a curable composition;
and
(4) activating said curing system and said blowing agent so that the
blowing of said material commences no sooner than when curing commences,
and before said terpolymer has been fully cured;
wherein the ratio of said terpolymer to said polypropylene, on a weight
basis, is from about 1:1 to about 3:1, and
wherein a chemical blowing agent activator is added during step (3).
11. A process according to claim 10 wherein said curing system is activated
and said material is partially cured before said blowing agent is
activated.
Description
TECHNICAL FIELD
This invention relates to foamed blends of polymeric materials. More
particularly, this invention relates to blends of thermoplastic polymers
with elastomeric polymers, blown with chemical blowing agents.
Specifically, this invention relates to chemically blown blends of
thermoplastic polymers with desirably cured elastomeric polymers in which
the cure-produced cross-linking present in the elastomers during blowing
is such that the modulus of the elastomers is sufficiently high to prevent
escape of the gas bubbles produced during blowing, but not so high as to
exceed the level at which bubble formation in the elastomers would be
detrimentally impeded.
BACKGROUND OF THE INVENTION
While both thermoplastic materials and elastomers are widely used in
industrial and domestic applications, certain "hybrid" blended polymers
displaying the attributes of both have come to be recognized as more
desirable than either in certain applications, particularly in the
automotive field. These blended polymers are generically known as
thermoplastic elastomers.
A group of these materials, sometimes termed polyolefinic thermoplastic
elastomers, POTPE, avoids the disadvantage of requiring rubber machinery
for processing, but may be processed instead on conventional plastic
processing equipment. Thus, while the materials display rubber-like
properties at room temperature, they become plastic at elevated
temperatures. This characteristic, for example, allows the blended
materials to be readily extruded, injection molded, and otherwise worked
without any need for excessive cycle times, specialized machinery, or
other costly and often inconvenient processing expedients.
Among other applications, polyolefinic thermoplastic elastomers have been
widely used in the form of foamed materials. Such products, for instance,
are readily foamed during the process of extrusion by being injected with
chlorofluorocarbons such as, for example, Freon 11, marketed by the EI
DuPont Company of Wilmington, Del., and foamed products exhibiting
desirably low densities, that is, in the order of 0.4 gms. per cc, have
been formed in this way.
The use of chlorofluorocarbons has been linked by some with damage to the
earth's ozone layer, however, and attempts have been made to find
substitutes for such blowing agents. In this connection, modified forms of
chlorofluorocarbons, i.e., hydrochlorofluorocarbons, HCFCs, have been
suggested as possible substitutes for chlorofluorocarbons; however, these
materials are still under development and their usefulness as replacement
materials is still not certain. Furthermore, injection techniques require
elaborate apparatus and necessitate careful monitoring, thus they are
burdensome for those reasons as well.
Another approach to foaming has involved the use of so-called "chemical
blowing agents", i.e., substances that decompose upon being heated to
their decomposition temperature, producing desired blowing gases, such as
nitrogen. Such materials pose no threat to the environment. However, when
used with polyolefinic thermoplastic elastomers such as, for example,
Santoprene, a POTPE marketed by Monsanto Company of Saint Louis, Missouri,
the chemical blowing agents appear unable to produce foamed products with
densities as low as those achievable with the chlorofluorocarbons. In this
regard, they appear able only to effect marginal density reductions, that
is, in the order of 15% to 20%. In any event, there is considerable
potential advantage in finding suitable techniques for blowing
polyolefinic thermoplastic elastomers without employing
chlorofluorocarbons, but in a way in which blends exhibiting desirably low
densities can be achieved. Such advantage includes the fact that POTPEs
can be processed like thermoplastics, while exhibiting the resilience and
elasticity of elastomers. In the automotive field, for example, such
materials lend themselves to uses including weather stripping for
automobiles, due to the excellent sealing ability made possible as a
consequence of their softness.
DESCRIPTION OF THE INVENTION
In view of the preceding, therefore, it is a first aspect of this invention
to prepare foamed blends of thermoplastic polymers with elastomeric
polymers.
It is a second aspect of this invention to prepare foamed blends of
thermoplastic polymers with elastomeric polymers, utilizing chemical
blowing agents.
It is an additional aspect of this invention to provide foamed blends of
thermoplastic polymers with elastomeric polymers, characterized by
relatively low densities of the blends.
Another aspect of this invention is to provide a process in which foamed
blends of thermoplastic polymers with elastomeric polymers having
densities substantially as low as those obtainable using
chlorofluorocarbons can instead be prepared with chemical blowing agents.
It is a further aspect of this invention to limit the degree of
cross-linking of elastomeric polymers contained in blends with
thermoplastic polymers during the foaming of the blends by means of
chemical blowing agents.
Yet another aspect of this invention is to provide a process for blowing
blends of thermoplastic polymers with elastomeric polymers in which the
blowing occurs in both phases of the polymeric blends.
Still a further aspect of this invention is to provide both a one-step, and
a two-step process for preparing foamed blends of thermoplastic polymers
with elastomeric polymers, using chemical blowing agents.
The foregoing and additional aspects of this invention are provided by a
process for preparing a foamed polymeric material from a blend of a
thermoplastic polymer with an elastomeric polymer comprising the following
steps: (1) mixing a chemical blowing agent and rubber compounding
additives with said elastomeric polymer to form a mixture; (2) mixing said
mixture with a thermoplastic polymer at a temperature at which at least
one of said polymers is in a fluid state, thereby forming a homogeneous
blend; (3) mixing said blend with a curing system to form a curable
composition; and (4) activating said curing system and said blowing agent
so that the blowing of said material commences no sooner than when curing
commences and before said elastomeric polymer has been fully cured.
The foregoing and other aspects of this invention are provided by a foamed
polymer blend prepared by the process of the preceding paragraph.
The foregoing and further aspects of this invention are provided by a
process for preparing a foamed polymeric material from a blend of
polypropylene and a ethylene-propylene terpolymer comprising: (1) mixing a
chemical blowing agent and rubber compounding additives with said
ethylene-propylene terpolymer to form a mixture; (2) mixing said mixture
with polypropylene at a temperature at which at least one of said polymers
is in a fluid state thereby to form a homogeneous blend; (3) mixing said
blend with a curing system to form a curable composition; and (4)
activating said curing system and said blowing agent so that the blowing
of said material commences no sooner than when curing commences and before
said terpolymer has been fully cured, wherein the ratio of the resins,
i.e., terpolymer to said polypropylene, on a weight basis, is from about
1:1 to about 3:1, and wherein a chemical blowing agent activator is added
during step (3).
The foregoing and still additional aspects of this invention are provided
by a foamed polymer blend prepared by the process of the preceding
paragraph.
BRIEF DESCRIPTION OF THE INVENTION
The invention will be better understood when reference is had to FIG. 1,
showing a schematic graph plotting the elastomeric modulus versus the
degree of elastomeric cure, with differences in blowing timing illustrated
thereon.
DETAILED DESCRIPTION OF THE INVENTION
While not wishing to be bound by theory, physical evidence suggests that
foaming in the thermoplastic phase of blends containing both thermoplastic
and cured elastomeric compounds can be achieved with little difficulty.
However, foaming of the elastomeric phase appears to be considerably more
difficult. For this reason, it is hard to obtain desirable low densities
in blends containing a relatively high proportion of a cured elastomer
since foaming appears to occur primarily in the thermoplastic polymer
present. This phenomenon makes it difficult to realize the improved
elastic sealing qualities one would expect to find in a composition
containing a foamed elastomeric polymer.
It is thought that the resistance of cured elastomers to foaming is the
result of undesirably high modulus characteristics. In this regard, it is
surmised that to be successfully blown, a cured elastomeric polymer
requires a "balance" of modulus. By way of explanation, if the level of
cross-linking is too high when the chemical blowing agent is activated,
the modulus of the elastomeric phase is probably too high to favor growth
of the gas bubbles produced by the blowing agent, thereby leading to
inferior foaming. On the other hand, if the degree of cross-linking is too
low, resulting in an elastomeric phase having a low modulus, the gas
bubbles formed tend to expand and escape, again producing inferior
blowing. In other words, there appears to be an optimum cross-linking
level between the two extremes, i.e., where the modulus of the elastomer
is low enough to permit optimal bubble growth, but high enough to retain
the bubbles, making it possible to produce a foamed elastomer in which the
bubbles are uniformly distributed throughout the elastomeric phase. In the
case of thermoplastic/elastomeric blends contemplated by this invention,
it has been found that by controlling the curing of the elastomeric phase
of the blends, blowing can be successfully achieved in both the
elastomeric and thermoplastic phases, permitting foamed blends with
desirably low densities to be achieved.
The effect of the phenomenon is demonstrated by the fact that when an
attempt is made to employ chemical blowing agents with blends of
thermoplastic polymers and fully cured elastomeric polymers in which the
thermoplastic constituent is present to an extent of about 70%, by weight,
in other words a relatively hard blend, a 100% expansion, by volume, of
the blend can be achieved during the blowing process. When, however, the
thermoplastic constituent is present in amount of about 12-15%, by weight,
a softer blend, expansions of only about 15-25%, by volume, are
achievable.
FIG. 1 is a schematic graph illustrating the effect of modulus, i.e., the
extent of curing, with timing of the blowing process. In the Figure,
modulus of the elastomer is plotted against increasing cure time. Three
different points are shown on the curve at which the blowing process is
carried out. Thus, 1 in the FIGURE, shows the case in which blowing occurs
at a relatively low modulus. This allows escape of the blowing bubbles
from the elastomer and results in a roughened surface, reflecting the
bursting of the bubbles through the skin of the material. Point 3 on the
other hand, illustrates the case where blowing is carried out essentially
after the elastomer is fully cured. In this situation, there is little
bubble formation, and therefore, minimal expansion, i.e., little density
reduction. Point 2 on the other hand, illustrates the point at which
blowing of the elastomer is conducted after sufficient modulus to prevent
escape of the bubbles has developed during the curing process, but where
the modulus is still low enough to permit bubble growth and retention, the
optimum situation.
In view of the preceding, therefore, the invention includes the concept of
preparing curable blend compositions of thermoplastic polymers and
elastomeric polymers, particularly blend compositions containing
relatively high amounts of elastomeric constituents. The blend
compositions, which are compounded to contain both curing systems and
blowing agents are then cured and blown, the blowing sequence being timed
so that blowing is occurring after curing has begun, in a preferred mode,
after 75% to 80% of the curing has taken place. Several techniques can be
employed to achieve the low density foams that the invention is capable of
producing, i.e., those having densities as low as 0.5 gms. per cc to about
0.4 gms. per cc.
In one embodiment of the invention, sometimes termed a two-step, or
"static" process, a curable polymer blend compound containing a
thermoplastic polymer and an elastomeric polymer, and including a blowing
agent that is activated at a higher temperature than the curing system, is
cured to a desired point in an initial step carried out at a temperature
high enough to activate curing, but which is insufficient to initiate
blowing. In a second step thereafter, the composition is heated to the
activation temperature of the blowing agent, producing the blown polymer
blend.
In a second technique, both curing and blowing of the curable polymer blend
composition are carried out substantially simultaneously in a one-step, or
"reactive" process. In some variations of the one-step process, while some
bubble formation may occur prior to optimal cross-linking, sufficient
blowing occurs thereafter, but before too high a modulus is attained, to
produce a superior foamed blend product.
The broader aspects of the invention are illustrated in the following
process flow diagram.
##STR1##
Referring to the diagram, the preparation of the elastomeric polymer
compound involves the incorporation of various compounding additives and a
chemical blowing agent with a selected elastomeric polymer.
Various polymers may be used for the purpose including natural rubber, and
synthetic rubbers such as olefin based elastomers including
ethylene-propylene terpolymers (EPDM), nitrile rubbers, SBR, polybutylene
and others. In a preferred embodiment, however, EDPM rubber is employed.
Added at this point in the process are rubber compounding additives such
as, for instance, those well known in the art including processing oils,
non-reinforcing fillers, zinc oxide, stearic acid, lubricants, processing
aids, and the like.
Also at this point, a high temperature blowing agent is added to the
elastomeric polymer compound. The use of high-temperature blowing agents
is necessary in order to avoid premature activation of the agents during
incorporation of the thermoplastic polymer selected, as will be apparent
from the discussion below. Such blowing agents include, for example, those
activated at from about 150.degree. C. to about 260.degree. C. An example
of an additive of this type is p-toluene sulfonyl semicarbazide, marketed
by Uniroyal Chemicals under the trademark Celogen RA, which has a
decomposition temperature ranging from 216.degree. C. to 260.degree. C.
Another similar material is azodicarbonamide, also marketed by Uniroyal
Chemicals under the trademark Celogen AZ 130. The latter compound
decomposes at from about 199.degree. C. to 232.degree. C. Other similar
high-temperature blowing agents of the types known to those skilled in the
art including, for example, 5-phenyltetrazole, isatoic anhydride, and
trihydrazino-triazine may also be used. In some cases, auxilliary blowing
agents in very small quantities may also be added for
initiation/nucleation, examples of such auxilliary chemical blowing agents
being azodicarbonamide; diphenyl sulfone-3,3'-disulfohydrazide;
4-4'-oxybis(benzenesulfohydrazide), and di-phenylene
oxide-4-4'-disulfohydrazide.
Incorporation of the above-described components may readily be
accomplished, for instance, in extruding equipment, roll mills, internal
mixers, static mixers, or similar devices.
The time required for obtaining a uniform mixture will depend upon the
relative amounts of the components present, their specific nature, the
temperature at which the mixing operation is conducted, as well as the
equipment employed, and other variables. In any event, mixing should be
carried out at a temperature below that at which activation of the blowing
agent would occur. Typically, for example, mixing is carried out at about
120.degree. C. for a period sufficient to assure that a uniform mixture is
obtained.
Following preparation of the elastomeric polymer compound as indicated, a
polymer blend is prepared by adding a selected thermoplastic polymer to
the elastomeric polymer compound at a temperature sufficiently high to
fluidize at least one of the polymers forming the blend.
Among thermoplastic polymers desirable for purposes of the invention may be
mentioned polyolefin polymers such as polypropylene, polyethylene and
poly(4-methyl pentene-1) as well as polymers such as polystyrene,
poly(methyl methacrylate), poly (vinylidene chloride) and others. However,
in a preferred embodiment of the invention, polypropylene is employed.
Any of various ratios of the elastomeric polymer to the thermoplastic
polymer may be employed, depending upon the nature of the blown product
required. It has been found however that blends in which the ratio is
adjusted from about 1:1 to about 3:1, elastomer to thermoplastic polymer,
provide particularly soft, low density polymer blends characterized by
excellent sealing characteristics. The use of such ratios, equivalent to
blends containing from about 50% to 75% elastomer, is therefore
particularly preferred.
In order to obtain superior homogeneity of the polymers making up the
blends, it is necessary that the mixing be conducted at a temperature
sufficiently high to fluidize at least one of the polymers. In other
words, the temperature must be high enough to transform at least one of
the polymers into a fluid state. When the material thus becomes a viscous
liquid, as for instance above its Crystalline Melting Point, a homogeneous
polymer blend is obtained. The temperature at which the blending is
carried out, however, must not be high enough to activate the blowing
agent present since this would result in premature blowing and loss of
formed bubbles due to the relatively low modulus of the elastomeric
polymer at this point in the process. The optimum temperature will depend
upon the nature of the polymers being blended, as well as other factors,
and is readily determined by simple experimentation.
In the case of blending polypropylene with EPDM, for example, where a
Celogen RA blowing agent is present, the blending is carried in equipment
of the type previously described at a temperature of about 165.degree. C.
to about 180.degree. C.
If desired, additional blowing agent can be introduced at this point to
assure that the thermoplastic polymer added will contain a sufficient
amount of blowing agent, or the additional blowing agent can be added to
the thermoplastic polymer before it is blended with the elastomeric
polymer. Normally, however, a sufficient amount of blowing agent will have
been incorporated with the elastomer to migrate into the thermoplastic
polymer during the blending procedure. The required amount of blowing
agent to be added in the procedures outlined will depend upon the nature
of the blowing agent, as well as the kind of polymers making up the blend,
as well as the nature of the blown product desired, and will be determined
be empirically.
Following preparation of the polymer blend detailed in the preceding,
suitable curing system components, and preferably a chemical blowing agent
activator, are added to the blend at a temperature sufficiently low to
avoid initiation of curing. Suitable activator compounds include those
known in the art, comprising various polyols, ureas, organic acids and
bases, as well as large numbers of metallic compounds, particularly basic
lead, zinc and/or cadmium. The chemical blowing agent p-toluene sulfonyl
semicarbazide, for example, can be activated strongly by urea, dibasic
lead phthalate, stearate, phosphite, etc., or it can be mildly activated
by zinc oxide, stearic acid, zinc stearate, or barium stearate.
Any of the curing systems of the types known in the rubber industry may be
employed to cross-link the elastomeric polymer, for instance, those
including sulfur, tetramethylthiuram disulfide (TMTD),
2-mercaptobenzothiazyl disulfide (MBTS), and equivalent systems.
In addition, in a particularly preferred embodiment of the invention, a
chemical blowing agent activator is added at this point. Such activators
are well known in the art, being exemplified by BIK OT marketed by
Uniroyal Chemicals. The proper amount of activator required will depend
upon the nature and amount of the chemical blowing agent, and can readily
be determined by simple trial. Such activators not only enhance the action
of the chemical blowing agents, but allow blowing to occur at lower
temperatures, thus avoiding the discoloration that sometimes occurs when
such blends are heated to the normal activation points of their blowing
agents.
The amount of the curing components required will depend upon the degree of
cross-linking needed to obtain the modulus necessary to achieve optimal
blowing. This in turn will depend upon the nature of the elastomer being
used, as well as the type of product which it is desired to achieve.
Again, simple experimentation is employed to determine the amount
necessary.
In order to avoid premature curing of the curable composition thus
prepared, the mixing of the ingredients required to form the composition
will be carried out, for example, in equipment of the type described, at a
temperature low enough to avoid activation of the curing system. While
this will depend upon the nature of the curing system, commonly, the
mixing will be carried out a temperature no higher than about 130.degree.
C. until the ingredients are uniformly dispersed throughout the
composition.
Following preparation of the curable composition as aforesaid, typically
containing about 30% to 70%, by weight, of the resins, the blend may be
blown in either a one-step static process, or in a two-step, reactive
process.
In the static process, the curable blend composition is initially cured at
a temperature high enough to activate the curing system, but lower than
that required to activate the chemical blowing agent. Cure of the
elastomer then proceeds to a point predetermined by the amount of curing
components added, such amount being that previously found to produce
cross-linking which results in a modulus sufficient to permit bubble
formation during the subsequent blowing operation, but which is
insufficient to significantly impair formation of the bubbles.
Thereafter, the temperature of the blend containing the partially cured
elastomer is increased to the activation temperature of the blowing agent,
thus initiating the blowing that results in the final foamed blend
product. The temperatures required for each of the steps will depend upon
the nature of the curing system and the chemical blowing agent used.
Within such considerations, commonly the curing step will proceed at a
temperature of from about 160.degree. C. to 170.degree. C. for 10 to 15
minutes, while the blowing process will take place at about 210.degree. C.
to 235.degree. C. for about 5 minutes.
The reactive process lends itself to extrusion of the curable composition,
a process that results in the substantially simultaneous curing and
blowing of the curable composition. The extruders may be equipped with
controllable "heating zones" along their barrels. This permits their
adjustment to obtain the desired sequence of curing and blowing activation
so that the bubbles formed are generated at an optimal modulus.
Alternatively, the reactive process may be carried out in a pressure mold
maintained at a temperature sufficient to activate both the curing system
and the chemical blowing agent. In such a case, curing will proceed
substantially immediately, while the chemical blowing agent will require
an induction period to activate. This difference in effective activation
time provides a delay sufficient to develop enough curing to provide the
modulus necessary for proper blowing.
If desired, combinations of different chemical blowing agents can be
employed to vary the speed of blowing. While several blowing options are
available as described in the preceding, the foam densities resulting from
the procedures are quite similar.
The following example, while not intended to be limiting in nature, is
illustrative of the invention.
In a first step, EPDM, a processing oil, non-reinforcing fillers, zinc
oxide, stearic acid, Celogen RA, wax, lubricant, and a processing aid are
mixed on a two-roll mill with the roll temperature maintained at
120.degree. C.
In a second step, the above mixture is blended with polyeropylene a
two-roll mill with the roll temperature maintained at 180.degree. C.
Ordinarily because of the high decomposition temperature of the chemical
blowing agent used, a processing temperature of 220.degree. C. or higher
would be required for full decomposition of the chemical blowing agent.
The decomposition temperature, however, is significantly reduced by
incorporation of a surface treated urea, BIK OT, as an activator, added in
a third-step as follows.
The above blend is subsequently mixed with curatives including sulfur,
TMTD, MBTS, BIK OT, and Celogen AZ 130, added for cell nucleation, on a
two-roll mill with the roll temperature being maintained at 120.degree. C.
Table 1 lists the formulation of the blends prepared by the above
procedure.
TABLE 1
______________________________________
Compound No.
1 2 Control 3 4
______________________________________
Epsyn 70A.sup.1
100 100 100 100 100
Profax 6823.sup.2
39 39 35 35 35
Minstron Vapor
25 25 25 25 25
Talc.sup.3
Akrochem Allied
25 25 25 25 25
Whiting A-1.sup.4
Sunpar 2280.sup.5
120 120 100 120 120
Stearic Acid.sup.6
2 2 2 2 2
Zinc Oxide.sup.6
5 5 5 5 5
Carbowax 2 2 2 2 2
PEG 3350.sup.7
Struktol WB212.sup.8
2 2 2 2 2
Celogen RA.sup.9
3 3 -- 1 1
CelognAZ130.sup.9
1 1 -- 0.25 0.25
BIK OT.sup.9
-- 3 -- 1 1
Sulfur.sup.6
2 2 1 1 0.5
MBTS.sup.4,10
0.5 0.5 0.25 0.25 0.125
TMTD.sup.4,11
1.0 1.0 0.50 0.50 0.25
______________________________________
Supplied by:
.sup.1 Copolymer Rubber and Chemical Corp.
.sup.2 Himont U.S.A., Melt Flow Index = 0.4 gm/10 min
.sup.3 Cyprus Industrial Minerals Company
.sup.4 Akrochem Corporation
.sup.5 Sun Refining Company
.sup.6 Fisher Scientific
.sup.7 Harwick Chemical Corporation
.sup.8 Struktol Company
.sup.9 Uniroyal Chemical
.sup.10 2,2dithiobisbenzothiazole
.sup.11 Tetamethyl Thiuramdisulfide
The compounded blend is then compression molded at 75.degree. C. into 0.25
inch-thick sheets which are then foamed in an oven at different
temperatures and different residence times. Two techniques are employed as
follows.
(1) In a reactive process, the compression molded sheet is subjected to an
oven temperature higher than the chemical blowing agent decomposition
temperature to promote substantially simultaneous curing and blending.
(2) In a static process, the compression molded sheet is first cured at a
temperature lower than the chemical blowing agent decomposition
temperature. After this, the cured sheet is subjected to a temperature
higher than the decomposition temperature of the chemical blowing agent to
induce foaming.
Table 2, illustrating the one-step technique, shows the effectiveness of
the activator in promoting the decomposition of the chemical blowing agent
at a variety of temperatures and residence times.
TABLE 2
______________________________________
Oven Residence Compound Control
Compound
Temp. Time No. Density
No.
(.degree.C.)
(min) 1 2 (gm/cc)
3 4
______________________________________
Ambient -- -- -- 0.98 0.99 0.97
180 3 0.96 0.97 -- -- --
180 5 0.90 0.77 -- -- --
180 10 0.63 0.38 1.01 0.60 0.62
200 5 0.75 0.46 0.99 0.52 0.56
210 5 -- -- 1.00 0.49 0.50
______________________________________
Table 3 shows the two-step foaming technique in which the blends, having a
density of 0.97-0.99 gm/cc before any heating, were first cured at
165.degree. C. to 180.degree. C. for from 5 to 10 minutes, and then foamed
at 210.degree. C. for 5 minutes.
TABLE 3
______________________________________
Compound No.
3 4
Cure Temp. (.degree.C.)
Cure Time (min)
Density (gm/cc)
______________________________________
165 5 0.87 0.82
165 10 0.81 0.77
180 5 0.71 0.72
180 10 0.60 0.62
______________________________________
While in accordance with the patent statutes, a preferred embodiment and
best mode has been presented, the scope of the invention is not limited
thereto, but rather is measured by the scope of the attached claims.
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